The Collaborative International Dictionary of English v.0.48:

Prolong \Pro*long"\, v. t. [imp. & p. p. Prolonged; p. pr. &
   vb. n. Prolonging.] [F. prolonger, L. prolongare; pro
   before, forth + longus long. See Long, a., and cf.
   Prolongate, Purloin. ]
   [1913 Webster]
   1. To extend in space or length; as, to prolong a line.
      [1913 Webster]

   2. To lengthen in time; to extend the duration of; to draw
      out; to continue; as, to prolong one's days.
      [1913 Webster]

            Prolong awhile the traitor's life.    --Shak.
      [1913 Webster]

            The unhappy queen with talk prolonged the night.
                                                  --Dryden.
      [1913 Webster]

   3. To put off to a distant time; to postpone. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prolong
    v 1: lengthen in time; cause to be or last longer; "We prolonged
         our stay"; "She extended her visit by another day"; "The
         meeting was drawn out until midnight" [syn: prolong,
         protract, extend, draw out]
    2: lengthen or extend in duration or space; "We sustained the
       diplomatic negotiations as long as possible"; "prolong the
       treatment of the patient"; "keep up the good work" [syn:
       prolong, sustain, keep up]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "prolong":
   adjourn, continue, dawdle, defer, delay, drag on, drag out, draw,
   draw out, dwell on, elongate, endure, extend, hang fire, hang up,
   hold, hold off, hold over, hold up, keep, keep alive, keep going,
   keep up, last, lay aside, lay by, lay over, lengthen, lengthen out,
   let out, linger on, maintain, perpetuate, persist, pigeonhole,
   postpone, preserve, procrastinate, produce, prolongate, prorogate,
   prorogue, protract, pull, push aside, put aside, put off,
   put on ice, recess, reserve, retain, set aside, set by, shelve,
   shift off, sleep on, spin out, stand over, stave off, stay, strain,
   stretch, stretch out, string out, suspend, sustain, table,
   take a recess, tauten, temporize, tense, tighten, waive